1. Field of the Invention
The present invention relates to emission microscopy and, more particularly, to integrated circuit inspection systems.
2. Description of Related Art
In the design of semiconductor devices, including integrated circuits (IC's) and the like, it is often desired to analyze current flow through various circuits. Such analysis can, for example, be undertaken to isolate points of potential failure.
Several types of analysis of current flow through circuits take advantage of the electroluminescent characteristics of silicon. It is well known, for example, that a semiconductor device will, under excitation, emit a small amount of light. Whole electron pairs associated with various defects recombine, and in that process emit photons. Avalanche breakdown, in particular, can be analyzed by observing emitted light. Light emission in an avalanche breakdown situation enables the detection and location of areas of current flow. Oxide defects also can be detected by observing the light emitted upon application of current. By observing the emitted light, the points of failure of a damaged product can be determined, and an analysis of design flaws and/or processing flaws can be undertaken.
All of the foregoing is discussed in greater detail in U.S. Pat. No. 4,680,635. Also discussed in that patent is the use of light emissions to determine the profile and detailed effects of an ESD (electrostatic discharge) event in an integrated circuit. During ESD, p-n junctions become forward biased or even go into avalanche breakdown. Light is emitted in either case. By capturing details regarding the emitted light pattern, aspects of the ESD event can be observed and the area of dissipation of the ESD energy determined.
Study of light emitted by semiconductor devices is rendered somewhat difficult because of the small amounts of light involved. Only about 0.01% of the whole electron pairs in silicon recombine by emitting a photon. Thus, hundreds of milliamperes of current must be applied to a silicon device in avalanche breakdown to view light emitted with the naked, human eye. As application of such high currents is generally not feasible, or ultimately productive, other methods of capturing information regarding emitted light have been developed. Heretofore, for example, light emitted from integrated circuits has been recorded with time exposure photography. A shortcoming of this method, discussed in U.S. Pat. No. 4,680,635, is inability to isolate time as a factor in the light emission. That is to say, steady state conditions, but not transient conditions, can be observed by this method. Accordingly, failure mechanisms that can only be observed in the transient state, such as the hot electron effect in inverters, cannot be observed.
Also in the past light emissions from integrated circuits have been observed using infrared or optical microscopes. Shortcomings of such microscopes for this use are inability to resolve time varying effects (as discussed in the immediately preceding paragraph) and inability to detect faint and/or subtle contrasts in light emissions.
A third past method for observing light emission involves emission microscopes. Prior art details regarding such microscopes, as well as further information regarding the other analysis techniques discussed above in this description of related art section, may be found in the related case, as well as in U.S. Pat. Nos. 4,680,635, 4,755,874, and 4,811,090.
Prior art emission microscope systems, like the other past methods for analyzing circuits based upon light emissions, have a number of shortcomings and deficiencies. For example, the standard grade optical microscopes within prior art systems are not able to transmit light from the near ultraviolet to the infrared without a more than minimal loss of intensity. Although prior systems generally include filter wheels, these filter wheels are not automated and thus are not capable of "stepping through" each filter sequentially to accumulate a spectral graph. Prior art systems also suffer because of poor signal to noise ratios. The combination of an image intensifier and an electronically cooled CCD camera in prior art systems does not have the best quantum efficiency over a broad range of wavelengths presently attainable. Additionally, such a combination has a relatively poor signal to noise ratio. A number of other shortcomings of prior art systems are discussed in the related case.
Based upon all of the foregoing, it should be appreciated that prior art circuit inspection systems, even those incorporating state of the art emission microscopes, have a number of shortcomings and deficiencies that reduce their usefulness.